1. Field of the Invention
The present invention relates to a color cathode ray tube apparatus such as a color picture tube and, more particularly, to a color cathode ray tube apparatus using a dynamic focus scheme for correcting a deflection error caused by a magnetic field generated by a deflection yoke.
2. Description of the Related Art
In general, a color picture tube apparatus, as shown in FIG. 1, has an envelope constituted by a panel 1 and a funnel 2 integrally connected to the panel 1, and a phosphor screen 3 constituted by stripe-like or dot-like tricolor phosphor layers for emitting blue, green, and red beams is formed on the inner surface of the panel 1. A shadow mask 4 in which a large number of apertures are formed is arranged opposite to the phosphor screen inside the phosphor screen 3. On the other hand, an electron gun assembly 7 for emitting three electron beams 6B, 6G, and 6R is arranged in a neck 5 of the funnel 2. The three electron beams 6B, 6G, and 6R emitted from the electron gun assembly 7 are deflected by horizontal and vertical magnetic fields generated by a deflection unit 8 arranged outside the funnel 2, and the phosphor screen 3 is horizontally and vertically scanned through the shadow mask 4, thereby displaying a color image on the phosphor screen.
As the above color picture tube apparatus, an in-line type color picture tube apparatus in which an electron gun assembly for emitting three electron beams 6B, 6G, and 6R arranged in a line, constituted by the center beam 6G and the pair of side beams 6B and 6R, and passing through the same horizontal plane is used as the electron gun assembly 7 is known.
The electron gun assembly 7 generally comprises an electron beam generating section constituted by a cathode and a plurality of electrodes sequentially arranged adjacent to each other on the cathode, for controlling electron emission from the cathode and focusing the emitted electrons to form three electron beams 6B, 6G, and 6R and a main electron lens section constituted by a plurality of electrodes for focusing and converging the three electron beams 6B, 6G, and 6R obtained from the electron beam generating section on the phosphor screen 3.
In the above color picture tube apparatus, in order to obtain excellent image characteristics on the phosphor screen 3, the three electron beams 6B, 6G, and 6R emitted from the electron gun assembly 7 must be appropriately focused, and electron beams 6B, 6G, and 6R must be converged on the entire area of the phosphor screen 3.
Of the above required conditions, convergence of the electron beams 6B, 6G, and 6R, as described in U.S. Pat. No. 2,957,106, can be achieved by a method in which the three electron beams to be emitted from the electron gun assembly are inclined prior to the emission and then emitted. As described in U.S. Pat. No. 3,772,554, the following method is known. That is, of three electron beam through holes of each of the electrodes constituting the main electron lens section, a pair of side beam through holes are slightly shifted outside with respect to the side beam through holes of an adjacent electrode on the electron beam generating section side to converge the three electron beams. Both the methods are popularly, practically used.
However, even when the electron gun assembly 7 employing the above methods is incorporated in the cathode ray tube, in the actual color picture tube apparatus, when the electron beams are deflected, misconvergence of the three electron beams occurs. For this reason, a color picture tube apparatus having the following arrangement is known. That is, the deflection unit 8 generates a pin-cushion-shaped horizontal deflection magnetic field and a barrel-shaped vertical deflection magnetic field with respect to the three electron beams arranged in a line and passing through the same horizontal plane, and the three electron beams 6B, 6G, and 6R arranged in a line are converted on the entire area of the phosphor screen 3 by the deflection magnetic fields. This color picture tube apparatus is called a self-convergence in-line type color picture tube apparatus, and is dominant in color picture tube apparatuses at present.
However, when the three electron beams 6B, 6G, and 6R are converted by the deflection magnetic fields from the deflection unit 8 as described above, the electron beams 6B, 6G, and 6R considerably receive deflection errors, and distortion of the beam spot on the phosphor screen 3 increases, thereby causing a decrease in resolution. That is, as shown in FIG. 2 with respect to a horizontal deflection magnetic field, when an electron beam 6 is deflected to the right side of the drawing, the electron beam 6 receives a focusing effect by a pin-cushion-shaped horizontal deflection magnetic field 10 in a vertical direction (Y-axis) as indicated by an arrow 11. On the other hand, in a horizontal direction (X-axis), the magnetic flux densities on the right and left sides of the electron beam 6 are different from each other, and the magnetic flux density on the right side is higher than that on the left side. For this reason, the right side of the electron beam 6 receives a large deflection effect, and the electron beam 6 is horizontally drawn.
More specifically, the pin-cushion-shaped horizontal deflection magnetic field 10 works as a quadrupole lens for horizontally diverging and vertically focusing the electron beam 6, and has a prism effect for deflecting the electron beam 6. As a result, as shown in FIG. 3, a beam spot 13, at a peripheral portion of the screen, of the electron beam 6 deflected by the horizontal deflection magnetic field 10 is set in an over-focus state in the vertical direction, and low-luminance halo portions 15 are formed at the upper and lower portions of a high-luminance portion 14. Moreover, the beam spot 13 is set in an under-focus state in the horizontal direction and horizontally extends, and the resolution at the peripheral portion of the screen considerably decreases.
In order to prevent a decrease in resolution caused by a deflection error, in Jpn. Pat. Appln. KOKAI Publication Nos. 61-99249, 61-250934, or 2-72546, as shown in FIG. 4, electron gun assemblies having the following arrangement are disclosed. That is, first to fifth grids G1 to G5 are sequentially arranged along the traveling direction (the direction of the phosphor screen) of the electron beam 6, a predetermined DC voltage Vf is applied to the third grid G3, a voltage obtained by superposing a variable voltage Vd changed in accordance with the deflection amount of the electron beam 6 on the DC voltage Vf is applied to the fourth grid G4, and an anode voltage Eb is applied to the fifth grid G5.
In this electron gun assembly, when the above voltages Vf and Vd are applied, a quadrupole lens is formed between the third and fourth grids G3 and G4, and an end focusing lens is formed between the fourth and fifth grids G4 and G5. In the electron gun assemblies of the above publications, only the electrode structures are different from each other. The electron lenses which are basically equal to each other and have the same effects are formed in the electron gun assemblies, respectively.
FIG. 5 shows the above lenses using an optical model. In this optical model, an electron beam 6 emitted from the cathode passes through a quadrupole lens QL formed between the third and fourth grids G3 and G4, an end focusing lens EL formed between the fourth and fifth grids G4 and GS, an electron lens qL, and a prism pL of the deflection unit to reach the phosphor screen 3. When the electron beam 6 is directed to the center of the phosphor screen 3 without being deflected, the third and fourth grids G3 and G4 have almost equal potentials, and the quadrupole lens QL is not formed between the third and fourth grids. Therefore, the electron beam 6 is appropriately focused on the center of the phosphor screen 3 by the end focusing lens EL, and the beam spot 13 on the phosphor screen 3 has a circular shape.
In contrast to this, when the electron beam 6 is deflected, the potential of the fourth grid G4 increases in accordance with the deflection amount of the electron beam 6, the quadrupole lens QL is formed between the third and fourth grids G3 and G4, and, at the same time, the horizontal and vertical focusing effects of the end focusing lens EL between the fourth and fifth grids G4 and G5 are reduced. For this reason, as indicated by a broken line in FIG. 5, the electron beam 6 emitted from the electron gun assembly is set in an under-focus state in the vertical direction. However, since the electron beam 6 receives a focusing effect by a deflection error, i.e., an astigmatism, the electron beam 6 is appropriately focused in the vertical direction. On the other hand, the focusing effect of the quadrupole lens rarely changes in the horizontal direction, and the electron beam 6 is set in an under-focus state by the deflection magnetic field. However, since the distance between the peripheral portion of the phosphor screen 3 and the electron gun assembly is longer than that between the central portion and the electron gun assembly, the electron beam 6 is appropriately focused in the horizontal direction, and the beam spot 13 on the phosphor screen 3 has an almost circular shape.
However, when the electron beam 6 is focused by this dynamic focus scheme, the following problems are posed.
That is, a deflection error increases in accordance with an increase in size of the tube or an increase in deflection angle, and the vertical diverging effect of the quadrupole lens QL required for correcting this deflection error must be increased. As a result, since the horizontal focusing effect of the quadrupole lens QL increases, the focusing effect of the end focusing lens EL must be considerably reduced. For this reason, a potential difference between the electrodes required for reducing the focusing effect of the end focusing lens EL increases, and problems on safety or reliability, e.g., an increase in circuit load of a television set, discharge, or breakdown voltage are posed. As a more serious problem, the beam spot at the peripheral portion of the phosphor screen horizontally elongated shape. In this manner, when the horizontal size of the beam spot is larger than the vertical size, the horizontal resolution of the screen considerably decreases. In addition, when the vertical size of the beam spot becomes very small, a moire is formed due to the interference between the vertical size and the arrangement pitch of the apertures of the shadow mask, thereby degrading image quality.
A reason why the beam spot has a horizontally elongated shape will be is described as follows. That is, as shown in FIG. 5, the electron beam 6 emitted from the cathode forms a crossover, is slightly pre-focused by a pre-focus lens formed by the second and third grids, is incident on an electron lens system at a divergent angle .alpha., and is focused at a focusing angle .beta.Hc in the horizontal direction and at a focusing angle .beta.Vc in the vertical direction on the center of the phosphor screen 3. At this time, assuming that the potential of a crossover portion and the potential of the phosphor screen are represented by Vo and Vi, respectively, a horizontal image formation magnification MHc and a vertical image formation magnification MVc are represented by equations (1) and (2), respectively: EQU MHc=(.alpha./.beta.Hc)(Vo/Vi).sup.1/2 ( 1) EQU MVc=(.alpha./.beta.Vc)(Vo/Vi).sup.1/2 ( 2)
When the beam is to be focused on the center of the phosphor screen 3, the following equation is established: EQU .beta.Hc=.beta.Vc (3)
For this reason, the image formation magnifications MHc and MVc satisfy the following equation: EQU MHc=MVc (4)
and the beam spot at the center of the phosphor screen 3 has a circular shape.
However, when the electron beam is deflected, the quadrupole lens qL of the deflection unit works, and the quadrupole lens QL for correcting the deflection error works. At this time, at the peripheral portion of the phosphor screen 3, when the electron beam is focused at a focusing angle .beta.Hp in the horizontal direction and at a focusing angle .beta.Vp in the vertical direction, a horizontal image formation magnification MHp and a vertical image formation magnification MVp are represented by equations (5) and (6): EQU MHp=(.alpha./.beta.Hp)(Vo/Vi).sup.1/2 ( 5) EQU MVp=(.alpha./.beta.Vp)(Vo/Vi).sup.1/2 ( 6)
When the electron beam is to be focused on the peripheral portion of the phosphor screen 3, the following condition is satisfied: EQU .beta.Hp&lt;.beta.Vp (7)
and the image formation magnifications MHp and MVp satisfy inequality (8). For this reason, at the peripheral portion of the phosphor screen 3, the beam spot has a horizontally elongated shape. EQU MHp&gt;MVp (8)
In order to decrease the horizontal size of the beam spot at the peripheral portion of the phosphor screen 3, the following electron gun assembly is disclosed in Jpn. Pat. Appln. KOKAI Publication Nos. 3-95835 and 3-93135. That is, in addition to the quadrupole and end focusing lenses of the electron gun assembly, an additional quadrupole lens is formed between the cathode and the above quadrupole lens, and the additional quadrupole lens is caused to have effects reverse to the focusing and diverging effects of the quadrupole lens of the electron gun assembly, thereby horizontally diverging and vertically focusing the electron beam. In this manner, the horizontal focusing angle .beta.Hp of the electron beam is made close to the focusing angle .beta.Vp in the vertical direction, and the image formation magnifications MHp and MVp are defined by equation (9); EQU MHp.apprxeq.MVp (9)
However, with the above technical means, as described in Television Society Technical Report IDY92-17, a divergent angle a of the electron beam in flowing a large current increases. For this reason, when the electron beam is further horizontally diverged by the additional quadrupole lens, the electron beam is largely influenced by a spherical aberration in the horizontal direction of the end focusing lens, and the size of the beam spot on the phosphor screen 3 does not theoretically decrease in the horizontal direction.
As described above, when a pin-cushion-shaped or barrel-shaped horizontal deflection magnetic field is generated by the deflection unit with respect to the three electron beams emitted from the electron gun assembly, passing on the same horizontal plane, and arranged in a line, the electron beams are influenced by the deflection error of the deflection magnetic field, and the beam spot on the peripheral portion of the phosphor screen is distorted, and the resolution considerably decreases.
As a technical means for solving the decrease in resolution caused by the deflection error, an electron gun assembly which uses a dynamic focus scheme and in which a quadrupole lens and an end focusing lens are formed along the traveling direction of an electron beam is conventionally known. However, in this electron gun assembly, the vertical diverging effect of the quadrupole lens for correcting the deflection error must be increased with an increase in size of the tube or an increase in deflection angle. In accordance with this, the horiziontal focusing effect of the quadrupole lens also increases, and the focusing effect of the end focusing lens must be considerably decreased. For this reason, the potential difference between the electrodes for forming the end focusing lens increases, and problems on safety or reliability, e.g., an increase in circuit load of a television set, discharge, or breakdown voltage are posed. Moreover, in the electron gun assembly, the beam spot at the peripheral portion of the phosphor screen has a horizontally elongated shape, the horizontal resolution of the screen decreases, and a moire is formed due to the interference between the vertical size and the arrangement pitch of the apertures of the shadow mask, thereby degrading image quality.
In order to solve the above problems, an electron gun assembly in which, in addition to the above quadrupole lens and the end focusing lens, another quadrupole lens is additionally formed between the cathode and the above quadrupole lens is known. However, when the quadrupole lens is additionally formed, the horizontal size of the beam spot on the phosphor screen does not theoretically decrease.